Orthopaedic implants, such as femoral implants, tibial implants, humeral implants, or others, can be installed in or otherwise associated with the bony anatomy for treating traumatic injuries, reconstructing joint function, or for other purposes. Such implants may include an elongated insertion region, such as the stem region of a femoral hip implant, which can be at least partially inserted into the medullary canal of the proximal femur.
In some instances, the success of the orthopaedic implant may depend on how well the elongated insertion region fits into the bony anatomy. For example, with a femoral hip stem, it is important that proximal portions of the elongated insertion region fit tightly into the medullary canal, such that the stem loads proximal portions of the femur, preventing bone loss through stress shielding and/or resorption (and potentially subsequent failure of the implant). It is also important that distal portions fit snugly into the medullary canal; however, the fit should not be so tight as to prevent proximal loading.
A good fit between the orthopaedic implant and its associated bony anatomy may also help to prevent or lessen micromotion between the implant and the bone. Excessive micromotion may also lead to implant failure.
Because bone geometries vary from person to person (and may also vary with age), typical orthopaedic implants are often offered as part of a set or series of different sized implants. Typically, implant sets are created by first designing one size of implant and then scaling that implant in a proportional manner to define the geometries of the other implant sizes (e.g., increasing the width of the elongated insertion region by a uniform amount along the entire length of the stem).
Typical implant system growth does not accurately reflect the geometries of different bone sizes. Larger femurs, for example, are not simply bigger versions of smaller femurs. For instance, it has been discovered that proximal portions of the medullary canal (some or all of which may be referred to as the metaphysis) may “grow” at a greater rate than distal portions (some or all of which may be referred to as the diaphysis) as femoral size increases. Thus, femoral hip stem sets that grow the proximal portion at the same rate as the distal portion from size to size do not necessarily reflect the actual geometries of the various sizes of femurs. Thus, implant sets made in accordance with traditional methodologies may, in some cases, fit poorly when installed, and may lead to implant failure for the reasons discussed above or for other reasons.